1. Field of the Invention
The present invention relates to a control apparatus for compressor with induction motor. More specifically, the present invention relates to an apparatus and method for controlling an induction motor which drives a compressor included in a refrigeration cycle.
2. Description of the Prior Art
In a refrigeration cycle, an induction motor which drives a compressor is controlled such that a frequency f of an alternating current electric power is changed according to a change of a load of the refrigeration cycle. At this time, a voltage v of the alternating current electric power is controlled in a manner that a value of v/f becomes always constant with respect to the change of the frequency f. Then, the value of v/f is set such that an operation efficiency of the compressor becomes maximum in a stable state of the compressor, that is, in a state where a slip of the induction motor exists within a predetermined range. Therefore, at a time that the load is changed, that is, at a time that the change of the frequency f is rapid, since the induction motor becomes a light-load or an overload, the slip of the induction motor is changed, and accordingly, a larger exciting current flows or a larger load current flows, and thus, the operation efficiency of the compressor becomes to be lowered.
One method for solving such a problem is disclosed in Japanese Patent Application Laying-open No. 61(1986)-20236 for example. In this prior art, a power factor of the alternating current electric power which is supplied to the induction motor is evaluated, and the voltage of the alternating current electric power is controlled such that the power factor becomes maximum. However, in the prior art, if a wave-form of the alternating current electric power supplied to the induction motor contains a distortion, especially, a distortion due to harmonic waves, detection accuracy of the power factor decreases and therefore, it is uncertain to expect that the operation efficiency is increased. More specifically, if the alternating current electric power obtained from a commercial power source is directly supplied to the induction motor, the detection accuracy of the power factor is good since the distortion contained in the wave-form of the alternating current electric power is small; however, if the alternating current electric power of a false sine-wave generated by an inverter circuit of a PWM system is supplied to the induction motor, although the false sine-wave is smoothed to some degree by an inductance component of the stator windings of the induction motor, the current wave-form still contains a distortion, and therefore, the operation efficiency can not be increased sufficiently. Especially in a case of an induction motor having a small output power (less than few kw), such a problem is magnified.
A further method is disclosed in Japanese Patent Application Laying-open No. 4(1992)-33584, for example. In the further prior art, the slip is detected on the basis of a wave-form of a current flowing in the induction motor, and it is intended to improve the operation efficiency by changing the voltage to be applied to the induction motor in a manner that the slip becomes a predetermined value. However, in such a case, if a distortion is contained in the current wave-form, the detection accuracy of the slip is decreased. Furthermore, a detection circuit for detecting the slip becomes complex.